Roadway communication system

ABSTRACT

Road transmission antennas ( 4   a,    4   b ) are disposed along a road, each radiating a single cell (E) with electromagnetic waves at the same frequency and of the same content. A vehicle mounted device ( 3 ) receiving the waves from the road transmission antennas ( 4   a,    4   b ) performs antenna pattern diversity reception. When entering a wave blocking area of a large vehicle, a small vehicle is provided with seamless communications with a stationary station.

FIELD OF THE INVENTION

[0001] The present invention is generally related to a roadwaycommunication system wherein a plurality of road antennas are disposedalong a road in a manner to define cells on the road, thereby providingmobile communications between a stationary station and a mobile station.

BACKGROUND OF THE INVENTION

[0002] Demand for communications between the road administration andground vehicles tends to grow even more in the future. Particularly,drivers on highways need frequent information exchanges between thestationary station and the mobile stations in order to drive under lessload and to prevent accidents. A developed form of such a system isexemplified by an automatic driving system with extensive provision ofvarious sensors and cameras on both the road and vehicle for closecommunications between the stationary station and vehicle (see, forexample, Japanese Unexamined Patent Publication No.8-241495 (1996).

[0003] The consideration of future expansion of the automatic drivingsystem dictates the need to construct a driver support system(hereinafter referred to as “roadway communication system”) based on thecommunications with vehicles. Preparatory to the construction of such asystem, communication areas (cells) must be provided over the roads.

[0004] It may be contemplated to lay leakage coaxial cables along theroads, which involves a large scale cable-laying work. Besides, theleakage coaxial cables need be embedded at a relatively great depth inthe roads and hence, the wave energy presents a disadvantageously smallrange with respect to a transverse direction of the lane.

[0005] In contrast, as shown in FIG. 28, a system with road antennas 120disposed at given space intervals along the road for communicationspermits one road antenna 120 to define a relatively broad cell 121. Inthis case, the road antennas are each connected to a respective controlstation of the road administration via an optical fiber, coaxial cableor the like.

[0006] (A) In a case where the road antennas are disposed, a largevehicle approaching a small vehicle may sometimes cut in a line of sightof the small vehicle. FIG. 29 illustrates a particular state where thesmall vehicle is in a wave blocking area of the large vehicle. Themicrowaves and millimeter waves at high frequencies have smalldiffraction angles and hence, are prone to be blocked. This results in abreakdown of roadway communications.

[0007] It is therefore, an object of the invention to provide a roadwaycommunication system accomplishing seamless communications between thestationary station and the mobile station.

[0008] (B) The installation of the road antennas generally entailsinter-carrier interference or inter-symbol interference associated withthe occurrence of delayed multipath waves, causative factors of whichare structures near the road or plural vehicles in the cell whichreflect the waves. When the inter-symbol interference occurs, a biterror rate is not improved even if the waves are received at highreception levels. This leads to a so-called floor error.

[0009] In the mobile communication system based on a single carrier, areceiver is generally equipped with an equalizer having inversecharacteristics of those of a transmission line thereby to eliminate theeffect of the inter-symbol interference associated with the delayedmultipath waves.

[0010] Unfortunately, the automobile travels through the cell at suchhigh speeds that the radio frequency energy field presents too sharpfluctuations per unit time for the equalizer to cope with thecalculations. Thus, it is impossible to transmit signals at less than agiven transmission error rate. Additionally, a large-scale hardware isrequired for implementing the equalizer, which results in great powerconsumption.

[0011] On this account, the invention has an object to provide a roadwaycommunication system capable of preventing the inter-carrierinterference and inter-symbol interference for accomplishing stablecommunications between the stationary station and the mobile station.

DISCLOSURE OF THE INVENTION

[0012] (1) A roadway communication system according to the invention forachieving the above object has an arrangement wherein a plurality ofroad transmission antennas are disposed at different places along aroad, each radiating the same cell with electromagnetic waves carried atthe same frequency and containing the same content, and wherein avehicle mounted device receiving the waves from the road transmissionantennas performs antenna pattern diversity reception (Claim 1).

[0013] According to the invention, the plural road transmission antennasradiate the waves based on signals modulated with data of the samecontent. In this case, the road transmission antennas each have aspecific directivity (including non-directivity) and therefore, thewaves from the road transmission antennas are incident on the vehiclealong different directions. Accordingly, when an incoming wave in onedirection of the vehicle is blocked by a large vehicle, the waveincident on the vehicle in another direction becomes relatively higherin reception level than the blocked wave. Hence, the vehicle receptionantennas performing the diversity reception can provide communicationsbetween the road transmission antennas and the vehicle mounted deviceeven when one of the waves is blocked.

[0014] Within the same cell, seamless communications are ensured betweenthe stationary station and the vehicle because the radiation to thevehicle is provided in plural different directions. This permits everyvehicle to receive road traffic information seamlessly and hence, theinventive roadway communication system is best-suited to the automaticdriving system, as well.

[0015] The diversity reception may be performed based on the receptionlevel of each directive wave received by the vehicle reception antenna(Claim 2).

[0016] The reception level of a wave received by the vehicle receptionantenna normally increases in value as the vehicle approaches a roadtransmission antenna radiating the wave. In a case where a wave to bereceived at the highest reception level is blocked by the large vehiclecutting in the transmission path thereof, for example, a reception levelof another wave from a different road transmission antenna may becomerelatively higher than that of the former wave. In consideration of thisevent, an arrangement may be made such that the reception levels of thewaves received by the vehicle reception antennas are directly detectedand the directivity of wave to be received is switched. Thus areattained more preferable communications.

[0017] The diversity reception may be accomplished by either of thefollowing operations: (a) an operation of switching or combining thesignals which were received by the vehicle reception antennas and are tobe decoded; and (b) an operation of switching or combining the codeswhich were received by the vehicle reception antennas and then decoded(Claim 3).

[0018] The vehicle reception antennas may be an array antenna, whereasthe vehicle mounted device may further comprise reception-signaldetection means for detecting a reception level or phase of the wavereceived by each of the vehicle reception antennas, so that thediversity reception means may perform the diversity reception usinginformation on the reception level or phase detected by thereception-signal detection means (Claim 4).

[0019] This configuration assumes a case where the array antenna oradaptive array antenna is applied to the vehicle reception antennas. Thephase control of the antenna provides a desired directivity.

[0020] If an optical fiber radio signal transmission system is used as atransmission system for supplying the signals to the transmissionantennas (Claim 5), no need exists for each road transmission antenna tohave a signal transmission unit with a frequency converter. This resultsin a simplified configuration of the road transmission antenna.

[0021] Orthogonal Frequency Division Multiplex (OFDM) modulationtechnique in which a guard time is provided at each symbol may be usedas a data modulation technique (Claim 6). OFDM system resistingmultipath effect is preferably applied to the inventive arrangementwherein the waves incident in different directions are received.Particularly, the provision of the guard time at each symbol iseffective to obviate the inter-symbol interference associated with delayin the multipath transmission.

[0022] The signal transmission from the plural road transmissionantennas to the same cell involves fear that a fractional difference incarrier frequency may occur between the transmission stations. It isknown that OFDM is more susceptible to the degradation of transmissionquality than other transmission systems. The optical fiber radio signaltransmission system provides a highly effective and economical solutionto this problem, ensuring that the carrier frequencies from the stationsare completely matched.

[0023] According to the invention for achieving the above object, aroadway communication system comprises a plurality of road receptionantennas for receiving electromagnetic waves radiated from a vehiclemounted device in different directions, the plural road receptionantennas being disposed in a manner to provide directivity to the samecell and adapted to perform diversity reception based on the signalsreceived the road reception antennas (Claim 7).

[0024] In the roadway communication system, the vehicle radiates thewaves in different directions thereby providing data to the stationarystation. The plural road reception antennas are so disposed as toprovide directivity to the same cell while the site diversity receptionis performed using the signals received by the respective roadtransmission antennas. As a result, the stationary station canpositively receive the waves despite the multipath effect on therespective radio frequency energy fields of the road reception antennas.

[0025] The diversity reception may be performed based on the receptionlevel of the wave received by the road reception antenna (Claim 8).

[0026] In a case where a wave to be received at the highest receptionlevel is blocked by the large vehicle cutting in the transmission paththereof, for example, a reception level of another wave from a differentroad transmission antenna may become relatively higher than that of theformer wave. In consideration of this event, an arrangement may be madesuch that the reception levels of the waves received by the roadreception antennas are detected and the reception antenna to receive thewave is changed. Thus are attained more preferable communications.

[0027] The diversity reception may be accomplished by either of thefollowing operations: (a) an operation of switching or combining thesignals which were received by the road reception antennas and are to bedecoded; and (b) an operation of switching or combining the codes whichwere received by the road reception antennas and then decoded (Claim 9).

[0028] If an optical fiber radio signal transmission system is used as atransmission system for receiving the signals from the road receptionantennas (Claim 10), no need exists for each road reception antenna toinclude a signal transmission unit with a frequency converter. Thisresults in a simplified configuration of the road reception antenna.

[0029] Orthogonal Frequency Division Multiplex (OFDM) modulationtechnique may be used as a data modulation technique (Claim 11). OFDMsystem resisting multipath effect is preferably applied to the inventivearrangement wherein the waves from the vehicle on road are received inan environment having high incidences of wave blocking by other vehiclesand wave reflections on surrounding structures. Particularly, theprovision of the guard time at each symbol is effective to obviate theinter-symbol interference associated with delay in the multipathtransmission.

[0030] (2) A roadway communication system according to the invention forachieving the above object has an arrangement wherein a plurality ofroad transmission antennas are disposed at different places along a roadand each radiate the same cell with waves carried at the same frequencyand containing the same content, wherein a position marker is disposedat or near the road for indicating a position on the road at whichreception levels of the waves radiated from the plural road transmissionantennas are switched, and wherein a vehicle mounted device receivingthe waves radiated from the road transmission antennas via vehiclereception antennas performs any one of the following operations inresponse to detection of the position marker at or near the road, theoperations including switching of directivities of the vehicle receptionantennas, and switching or combining the received signals (Claim 12).

[0031] In the roadway communication system, the position marker isdisposed at place on the road or the like for indicating the position onthe road at which the maximum reception levels are switched. In responseto the detection of arrival of the vehicle at the position marker, thedirectivities of the vehicle reception antennas are switched, or thereceived signals are switched or combined. Accordingly, the processingsbecome simpler than where the levels of the received signals aredetected and compared.

[0032] It is noted that “switching the directivities of the vehiclereception antennas using phase control” means reception phase controlfor directivity change when an array antenna is used as the vehiclereception antennas.

[0033] (3) A roadway communication system according to the invention forachieving the above object has an arrangement wherein a plurality ofroad transmission antennas are disposed at different places along aroad, each having a specific polarization characteristic and radiatingthe same cell with waves carried at the same frequency and containingthe same content, and wherein a vehicle mounted device receiving thewaves from the road transmission antennas performs polarizationdiversity reception (Claim 13).

[0034] According to the invention, the plural road transmission antennasradiate different polarization waves based on signals modulated withdata of the same content. In this case, the differently polarized waveshave different propagation characteristics and hence, are generallyreceived by the vehicle on road at different field strengths. Therefore,if a wave of one polarization characteristic is blocked by the largevehicle, the vehicle mounted device is allowed to receive a wave of theother polarization characteristic. By switching the polarizationcharacteristics of the vehicle reception antennas, the seamlesscommunications between the road transmission antennas and the vehiclemounted device are ensured even if the wave incoming from one side isblocked.

[0035] The vehicle mounted device may detect reception levels of thewaves received by the vehicle reception antennas on apolarization-characteristic basis so as to perform the diversityreception based on the reception level thus detected (Claim 14).

[0036] The reception level of the wave received by the vehicle receptionantenna normally increases in value as the vehicle approaches the roadtransmission antenna radiating the wave. In a case where a wave to bereceived at the highest reception level is blocked by the large vehiclecutting in the transmission path thereof, for example, a reception levelof another wave from a different road transmission antenna may becomerelatively higher than that of the former wave.

[0037] In consideration of this event, an arrangement may be made suchthat the reception levels of the waves received by the vehicle receptionantennas are directly detected and the reception polarizationcharacteristic is changed. Thus are attained more preferablecommunications.

[0038] The diversity reception may be accomplished by either of thefollowing operations: an operation of switching or combining the signalswhich were received by the vehicle reception antennas and are to bedecoded; and an operation of switching or combining the codes which werereceived by the vehicle reception antennas and then decoded (Claim 15).

[0039] The vehicle reception antennas may be a polarization arrayantenna, whereas the vehicle mounted device may further comprisereception-signal detection means for detecting a reception level orphase of the wave received by each of the vehicle reception antennas, sothat the diversity reception means may perform the diversity receptionusing information on the reception level or phase detected by thereception-signal detection means (Claim 16).

[0040] This arrangement assumes a case where the array antenna oradaptive array antenna is applied to the vehicle reception antennas. Thephase control of the antenna provides reception of a desiredpolarization wave.

[0041] The roadway communication system may further comprise a signaltransmission unit for transmitting signals modulated with data of thesame content to the road transmission antennas via a plurality oftransmission lines, and may use an optical fiber radio signaltransmission system as a transmission system for outputting the signalsto the plural transmission lines (Claim 17). According to the invention,the signal transmission unit supplies radio frequency signals to theroad transmission antennas through the optical fibers and therefore, noneed exists for each road transmission antenna to have a signaltransmission unit with a frequency converter. This results in asimplified configuration of the road transmission antenna.

[0042] Orthogonal Frequency Division Multiplex (OFDM) technique in whicha guard time is provided at each symbol may be used as a data modulationtechnique (Claim 18). OFDM system resisting multipath effect ispreferably applied to the inventive arrangement wherein the wavesincident in different directions are received. Particularly, theprovision of the guard time at each symbol is effective to obviate theinter-symbol interference associated with delay in the multipathtransmission.

[0043] The signal transmission from the plural road transmissionantennas to the same cell involves fear that a fractional difference incarrier frequency may occur between the transmission stations. It isknown that OFDM is more susceptible to the degradation of transmissionquality than other transmission systems. The optical fiber radio signaltransmission system provides a highly effective and economical solutionto this problem, ensuring that the carrier frequencies from the stationsare completely matched.

[0044] A roadway communication system according to the invention forachieving the above object comprises a plurality of road receptionantennas for receiving differently polarized waves from the vehiclemounted device, and has an arrangement wherein the plural road receptionantennas each have a specific polarization characteristic and are sodisposed as to provide directivity to the same cell, each performingdiversity reception based on the signals received by the road receptionantennas (Claim 19).

[0045] According to the invention, the vehicle supplies vehicle data tothe stationary station. The vehicle mounted device radiates waves ofdifferent polarization characteristics via the vehicle transmissionantennas. As a result, these waves are received by the road receptionantennas. The waves of the different polarization characteristics havedifferent propagation characteristics. Therefore, even if a wave of onepolarization characteristic is blocked by the large vehicle, the roadreception antenna is allowed to receive a wave of the other polarizationcharacteristic. Thus is ensured the seamless communications between thevehicle mounted device and the road reception antenna.

[0046] The roadway communication system may further comprisereception-level detection means for detecting reception levels of theplural road reception antennas on a polarization-characteristic basis,and the diversity reception means may perform the diversity receptionbased on the reception level detected by the reception-level detectionmeans (Claim 20).

[0047] In a case where the large vehicle moves toward a roadtransmission antenna to receive the polarized waves from the vehiclemounted device at the maximum reception level thereby to block thepolarized waves, for example, a reception level of another roadreception antenna at a different location may become relatively higherthan that of the former reception antenna. In consideration of thisevent, an arrangement may be made such that the reception levels of thewaves received by the road reception antennas are detected and thereception antenna to receive the wave is changed. Thus are attained morepreferable communications.

[0048] The diversity reception means may perform either of the followingoperations for diversity reception: an operation of switching orcombining the signals received by the road reception antennas; and anoperation of switching or combining the codes which were received by theroad reception antennas and then decoded (Claim 21).

[0049] The roadway communication system may further comprise a signalreception unit for receiving, via transmission lines, the signalsreceived by the road reception antennas, and may use an optical fiberradio signal transmission system as a transmission system for outputtingthe signals to the transmission lines (Claim 22).

[0050] According to the inventive arrangement, the signals received bythe road reception antennas may be outputted to the transmission linesas at high frequencies. A signal selection unit may readily compare thereceived signals at high frequencies. This results in a simplifiedconfiguration of the road reception antenna and signal selection unit.

[0051] The vehicle mounted device may use Orthogonal Frequency DivisionMultiplex (OFDM) modulation technique, as a data modulation technique,in which a guard time is provided at each symbol (Claim 23).

[0052] OFDM system resisting multipath effect is preferably applied tothe inventive arrangement wherein the waves from the vehicle on road arereceived in an environment having high incidences of wave blocking byother vehicles and wave reflections on surrounding structures.Particularly, the provision of the guard time at each symbol iseffective to obviate the inter-symbol interference associated with delayin the multipath transmission.

[0053] (4) A roadway communication system according to the invention forachieving the above object has an arrangement wherein a plurality ofroad transmission antennas are disposed at different places along aroad, each antenna having a specific polarization characteristic andradiating the same cell with the waves carried at the same frequency andcontaining the same content, wherein a position marker is disposed at ornear the road for indicating a position on the road at which receptionlevels of the waves from the road transmission antennas are switched,and wherein a vehicle mounted device comprises vehicle receptionantennas having different polarization characteristics for receiving thewaves radiated from the road transmission antennas, and marker detectionmeans for detecting an arrival of the vehicle at the position marker,and performs any one of the following operations in response to themarker detection means detecting the arrival of the vehicle at theposition marker, the operations including the switching of thepolarization characteristics of the vehicle reception antennas, theswitching of the received signals or codes, or the combining of thereceived signals or codes (Claim 24).

[0054] In the roadway communication system, the road marker is disposedat place, such as on the road, for indicating the position on the roadat which the maximum reception levels are switched. In response to thedetection of arrival of the vehicle at the position marker, thepolarization characteristic of the vehicle reception antenna is changed.Accordingly, the processings become simpler than where the levels of thereceived signals are detected and compared.

[0055] It is noted that “switching the polarization characteristics ofthe vehicle reception antennas using phase control” means receptionphase control for polarization-characteristic change when an arrayantenna is used as the vehicle reception antennas.

[0056] (5) A roadway communication system according to the invention forachieving the above object has an arrangement wherein a plurality ofroad transmission antennas are disposed at different places along a roadand each radiate the same cell with OFDM-modulated waves containing thesame content, and wherein a vehicle mounted device receives the wavesradiated from the road transmission antennas and demodulates thereceived waves (Claim 25).

[0057] In the roadway communication system, the plural road transmissionantennas radiate the waves based on signals OFDM-modulated with data ofthe same content. In this case, the waves from the road transmissionantennas are incident on the vehicle on road along different directions.Therefore, when a wave incoming in one direction is blocked by the largevehicle, the vehicle mounted device is allowed to receive a waveincident thereon in another direction, the reception level of whichbecomes relatively higher than the former wave. This ensures theseamless communications between the road transmission antennas and thevehicle mounted device.

[0058] OFDM system resisting multipath effect is preferably applied tothe inventive arrangement wherein the waves from the vehicle on the roadare received in an environment having high incidences of wave blockingby other vehicles and wave reflections on surrounding structures. As aresult, the communication quality is not degraded.

[0059] The roadway communication system of the invention may furthercomprise a signal transmission unit for transmitting signals modulatedwith data of the same content via a plurality of transmission lines tothe road transmission antennas, and may use an optical fiber radiosignal transmission system as a transmission system for outputting thesignals to the transmission lines (Claim 26).

[0060] In the inventive arrangement, the signal transmission unitsupplies the radio frequency signals to the road transmission antennasvia the optical fibers and therefore, no need exists for each roadtransmission antenna to have a signal transmission unit with a frequencyconverter. This results in a simplified configuration of the roadtransmission antenna.

[0061] It is preferred that Orthogonal Frequency Division Multiplex(OFDM) modulation technique in which a guard time is provided at eachsymbol is used as a data modulation technique (Claim 27). The provisionof the guard time at each symbol is effective to obviate theinter-symbol interference associated with delay in the multipathtransmission.

[0062] A roadway communication system according to the invention forachieving the above object has an arrangement wherein a vehicle mounteddevice radiates OFDM-modulated waves via a vehicle transmission antenna,and wherein the plural road reception antennas are disposed at differentplaces along a road as providing directivity to the same cell and eachperform demodulation using signals received by the road receptionantenna (Claim 28).

[0063] According to the invention, the vehicle supplies the vehicle datato the stationary station. In this case, the vehicle mounted deviceradiates the waves via the vehicle transmission antenna while thereception levels of the road reception antennas are affected by themultipath transmission. The invention employs the plural road receptionantennas and OFDM system resisting multipath effect, thereby ensuringpositive wave reception on the stationary station side and error-freedata recovery.

[0064] In the roadway communication system, the road reception antennasmay use an optical fiber radio signal transmission system for outputtingthe received signals to transmission lines to the road reception means(Claim 29). According to the inventive arrangement, the roadtransmission antennas supply the radio frequency signals to the roadreception means via the optical fibers and therefore, no need exists foreach road reception antenna to have a signal transmission unit with afrequency converter. This results in a simplified configuration of theroad reception antenna.

[0065] The vehicle mounted device may use OFDM modulation technique, asa data modulation technique, in which a guard time is provided at eachsymbol (Claim 30). The provision of the guard time at each symbol iseffective to obviate the inter-symbol interference associated with delayin the multipath transmission.

[0066] (6) The roadway communication system according to the inventionfor achieving the above object has an arrangement wherein the pluralroad transmission antennas each define an individual one of pluralsub-areas which are constituting a single cell (Claim 31).

[0067] According to the invention, the waves modulated with road trafficdata of the same content are individually incident on the respectivesub-areas so that the incoming direction of the wave changes each timethe vehicle transfers to the next sub-area. Accordingly, if the wave isblocked in one area, the vehicle moving to the next sub-area is allowedto receive the wave. This prevents the occurrence of a communicationbreakdown between the vehicle and the stationary station, ensuring theseamless communications.

[0068] Since the sub-area defined by each road transmission antenna isone fraction of a single cell, the road transmission antenna requires asmall transmission power. This results in reduced cost for the roadantennas.

[0069] In the case where the sub-areas are defined, as well, the wavesof the same frequency are simultaneously incident on the vehicle mounteddevice along different directions. Accordingly, it is preferred for thevehicle mounted device to select the wave of the maximum reception levelin order to avoid the fading effect.

[0070] (7) The roadway communication system according to the inventionfor achieving the above object has an arrangement wherein communicationsare carried out over a plurality of continuous cells, using signals atthe same frequency and of the same content (Claim 32).

[0071] This arrangement provides the vehicle mounted device with theseamless communications free from frequency switching (handover) duringthe travel of the vehicle, also contributing to a simplifiedconfiguration of the vehicle mounted device.

[0072] (8) The roadway communication system according to the inventionfor achieving the above object has an arrangement wherein the pluralroad transmission/reception antennas are disposed near a cell boundarywith respect to a longitudinal direction of the road (Claim 33).

[0073] In this arrangement, the road transmission antenna is locatednear a road transmission antenna of a neighboring cell and hence, thewaves radiated from these transmission antennas are incident on thevehicle on road along different directions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0074]FIG. 1 is a conceptual representation of an arrangement of aroadway communication system according to a first embodiment of theinvention;

[0075]FIG. 2 is a conceptual representation of an arrangement of avehicle mounted device;

[0076]FIG. 3 is a block diagram illustrating an exemplary electricalconfiguration of a stationary station;

[0077]FIG. 4 is a block diagram illustrating another exemplaryelectrical configuration of a reception unit in a base station;

[0078]FIG. 5 is a block diagram illustrating an exemplary electricalconfiguration of the vehicle mounted device;

[0079]FIG. 6 is a group of graphical representations of reception levelsof electromagnetic waves;

[0080]FIG. 7 is a graphical representation explanatory of a selectionprocedure of a reception level and a reception signal;

[0081]FIG. 8 is a diagram illustrating another exemplary road antennainstallation;

[0082]FIG. 9 is a block diagram illustrating a configuration of areception unit 46 when an adaptive array antenna is applied to a vehicleantenna 12;

[0083]FIG. 10 is a conceptual representation of an arrangement of theroadway communication system according to a second embodiment hereof;

[0084]FIG. 11 is a block diagram illustrating an electricalconfiguration of a reception unit in a vehicle mounted device;

[0085]FIG. 12 is a conceptual representation of the roadwaycommunication system according to a third embodiment hereof;

[0086]FIG. 13 is a conceptual representation of an exemplary arrangementof the vehicle mounted device;

[0087]FIG. 14 is a block diagram illustrating an exemplary electricalconfiguration of the stationary station;

[0088]FIG. 15 is a block diagram illustrating an exemplary electricalconfiguration of the vehicle mounted device;

[0089]FIG. 16 is a conceptual representation of the communication systemaccruing to a fourth embodiment hereof;

[0090]FIG. 17 is a block diagram illustrating an exemplary electricalconfiguration of the reception unit in the vehicle mounted deviceaccording to the fourth embodiment hereof;

[0091]FIG. 18 is a conceptual representation of an arrangement of theroadway communication system according to a fifth embodiment hereof;

[0092]FIG. 19 is a conceptual representation of an arrangement of thevehicle mounted device;

[0093]FIG. 20 is a block diagram illustrating an exemplary electricalconfiguration of the stationary station;

[0094]FIG. 21 is a graphical representation of symbol transmission basedon OFDM technique with frequency axis represented by “f” and time axisrepresented by “t”;

[0095]FIG. 22 is a block diagram illustrating another form of electricalconfiguration of the receiver in the stationary station;

[0096]FIG. 23 is a block diagram illustrating another exemplaryelectrical configuration of the vehicle mounted device;

[0097]FIG. 24 is a diagram illustrating another exemplary road antennainstallation;

[0098]FIG. 25 is a conceptual representation of an exemplary arrangementof the roadway system according to a sixth embodiment hereof;

[0099]FIG. 26 is a block diagram illustrating an exemplary configurationof the base station;

[0100]FIG. 27 is a block diagram illustrating an exemplary electricalconfiguration of the road antenna;

[0101]FIG. 28 is a conceptual representation of an arrangement of aconventional roadway communication system; and

[0102]FIG. 29 is a diagram explanatory of wave blocking in theconventional roadway communication system.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment (Directivity)

[0103]FIG. 1 is a conceptual representation of an arrangement of aroadway communication system according to a first embodiment of theinvention. The roadway communication system provides two-waycommunications between a stationary station 1 and a vehicle mounteddevice 3 on a vehicle 2.

[0104] In the stationary station 1, a plurality of cells E arecontinuously defined along a road. Near a boundary of individual cells Ewith respect to a longitudinal direction of the road, a first roadantenna 4 a and a second road antenna 4 b are installed, each having adirectivity toward each cell. The first and second road antennas 41, 4 beach radiate the cell E with electromagnetic waves of the same frequency(e.g., in 6 GHz band). More specifically, the first road antenna 4 aprovides radiation in a direction represented by the hollow arrow in thefigure whereas the second road antenna 4 b provides radiation in adirection represented by the solid arrow. Accordingly, theelectromagnetic waves of the same frequency are incident on any point inthe cell E in longitudinally forward and backward directions withrespect to the road. Hence, when passing through the cell E, the vehicle2 receives the electromagnetic waves incoming from front and from back.

[0105] It is noted that the road antenna 4 has a height above the ground“h” of, for example, 10 m whereas the cell E has a longitudinal length“r” of, for example, 100 m with respect to the road.

[0106] The two road antennas 4 a, 4 b are connected to a base station 6via optical fibers 5 a, 5 b, respectively. Each optical fiber 5 a, 5 bcomprises an up-cable and a down-cable. The optical fibers reduce signalattenuation as compared with a coaxial cable or the like used as thetransmission line and hence, the degradation of communication quality isprevented. As a matter of course, the optical fibers 5 a, 5 b may bereplaced by the coaxial cable.

[0107] The base station 6 modulates a signal using road trafficinformation and applies the resultant signal to the respective roadantennas 4 a, 4 b via the optical fibers 5 a, 5 b. Accordingly, thewaves radiated from the road antennas contain the same road trafficinformation.

[0108] The base station 6 receives vehicle data from the respective roadantennas 4 a, 4 b so as to perform suitable processings on the data. Thevehicle data (including vehicle ID data item and other data items onroad conditions detected by various unillustrated sensors) are obtainedby the vehicle mounted device 3 and sent therefrom via the roadantennas.

[0109]FIG. 2 is a conceptual representation of an arrangement of thevehicle mounted device 3. The vehicle mounted device 3 includes avehicle communication unit 11 and an vehicle antenna unit 12. Thevehicle communication unit 11 operates to radiate electromagnetic wavescontaining the vehicle data via the vehicle antenna unit 12. The vehiclecommunication unit 11 also obtains the road traffic data contained inthe waves radiated from the respective road antennas 4 a, 4 b andreceived by the vehicle antenna unit 12. The road traffic data thusobtained are supplied to a driver, for example.

[0110] The vehicle antenna unit 12 includes a pair of vehicle antennas12 a, 12 b mounted on a ceiling of the vehicle 2. The vehicle antennas12 a, 12 b are juxtaposed along an anteroposterior direction of thevehicle 2, each having a directivity along the anteroposterior directionof the vehicle 2. This permits the vehicle antennas 12 a, 12 b toreceive high level radiation from the road antennas 4 a, 4 b providingdirectivities thereto, respectively. On the other hand, the vehicleantennas 12 a, 12 b direct their radiations containing the vehicle datato the road antennas 4 a, 4 b.

[0111]FIG. 3 is a block diagram illustrating an electrical configurationof the stationary station 1. The base station 6 includes a transmissionunit 21 for supplying the road traffic data to the road antenna 4. Thetransmission unit 21 includes a modulation unit 22 modulating amodulation-carrier based on the road traffic data for generation of atransmission signal. The modulation unit 22 may employ QPSK as asuitable modulation technique but other modulation techniques such asQAM, BPSK, 8PSK and the like are applicable. Unless otherwise noted, thedescription herein is made on assumption that QPSK modulation is used.

[0112] The transmission signal is supplied to a mixer 23, whichgenerates a radio-transmission signal of 6 (GHz) band, for example, bycombining the transmission signal with a frequency-modulation carrierfrom a local oscillator 24. The radio transmission signal is amplifiedin a high-frequency amplifier 25 and then supplied to an electro-opticalconverter (E/O) 26 for direct conversion to an optical signal. Theresultant optical signal is outputted to the two up-optical cables 5 a,5 b. The optical signal is delivered to optic-electrical converters(O/E) 27 a, 27 b mounted to the respective road antennas 4 a, 4 b, whereit is converted back to the electrical signal for radiation via the roadantennas 4 a, 4 b.

[0113] When outputted to the optical fibers 5 a, 5 b, the optical signalconverted by the electro-optical converter (E/O) 26 must be distributed.An optical fiber coupler 59 is used for the distribution of the opticalsignals. The optical fiber coupler 59 of the known configuration isusable (for example, C-NS series commercially available from Fiber OpticCommunications Inc.).

[0114] The control station 6 further includes a reception unit 28 forobtaining the vehicle data from the road antennas 4 a, 4 b. When theroad antennas 4 a, 4 b receive the electromagnetic waves from thevehicle antennas 12 a, 12 b, the reception signals corresponding to thewaves are converted to optical signals by electro-optical converters(E/O) 29 a, 29 b. Subsequently, the resultant optical signals areoutputted to two down-cables 5 a, 5 b to be supplied to the receptionunit 28 of the base station 6.

[0115] The reception unit 28 includes two optic-electrical converters(O/E) 30 a, 30 b which convert the optical signals back to the originalreception signals. The reception signals are amplified in high-frequencyamplifiers 31 a, 31 b, respectively and then supplied to a switch unit32, such as comprised of a semiconductor switch. The amplified receptionsignals are also applied to a level comparator 33. The level comparator33 compares reception levels of the respective reception signals todetermine which of the reception signals is the higher. Then the levelcomparator provides control of the switch unit 32 for passage of areception signal of the maximum reception level.

[0116] In the configuration of FIG. 3, two signals are switched by theswitch unit 32. However, an alternative configuration may be made suchthat the two signals are weighted with a predetermined weighting factorand then combined. In this case, “the predetermined weighting factor” isdetermined based on the reception levels of the reception signalscompared by the level comparator 33.

[0117] According to the block diagram of FIG. 3, the switch unit 32operates to switch the high-frequency signals amplified by thehigh-frequency amplifiers 31 a, 31 b. An alternative configuration ispossible wherein data detected by a detector 36 are switched orcombined. Another configuration is also possible wherein data decoded bya decoder 37 are switched or combined.

[0118]FIG. 4 illustrates a configuration wherein the switch unit 32 isdisposed downstream of the detector so as to select a reception signalto be passed from reception signals subjected to coherent detection.More specifically, the reception signals amplified by high-frequencyamplifiers 31 a, 31 b are applied to mixers 34 a, 34 b to be changed infrequency. The resultant signals are subject to coherent detection indetectors 36 a, 36 b and then supplied to the switch unit 32. On theother hand, the level comparator 33 provides control of the switch unit32 for selective passage of a reception signal of the highest receptionlevel out of the reception signals amplified by the high-frequencyamplifiers 31 a, 31 b.

[0119] The selection of the reception signal subsequent to the detectionadvantageously provides signals less susceptible to noises, or signalquality deterioration.

[0120] The configuration where the selection of reception signal followsthe signal detection may be applied to a reception unit 46 of thevehicle communication unit 11 shown in FIG. 5.

[0121] As described with reference to FIGS. 3 and 4, a so-called opticalfiber radio signal transmission technique (A. J. Cooper, “FIBER/RADIOFOR THE PROVISION OF CORDLESS/MOBILE TELEPHONY SERVICES IN THE ACCESSNETWORK”, Electron.Lett., Vol.26, No.24 (November 1990) is used as atransmission technique for outputting the optical signal to the opticalfibers 5 a, 5 b.

[0122] This negates the need for mounting a transmission/reception unitto the respective road antennas 4 a, 4, permitting thetransmission/reception units for the antennas to take form as one setmounted to the base station 6. Thus, the road antennas 4 a, 4 b may beconstructed simple. On the other hand, the base station 6 may processthe reception signals fed from the road antennas 4 a, 4 b as they are athigh frequencies. Hence, the level comparator 33 may readily compare thehigh-frequency reception levels of the reception signals. This resultsin a simplified structure of the reception unit 28.

[0123] The base station 6 selects the reception signal based on aso-called site diversity system thereby accomplishing an accuratedecoding of the vehicle data in the subsequent processing.

[0124] In FIG. 3, the reception signal through the switch unit 32 goesto a mixer 34 where it is combined with a frequency-modulation carrieroutputted from a local oscillator 35 to be changed in frequency.Subsequently, the resultant signal is applied to the detector 36 to becoherently detected using a demodulation carrier. Then, the signal isapplied to the decoder 37 for conversion to a reception signalcorresponding to the vehicle data.

[0125] In order for the vehicle mounted device 3 to accomplish accuratedecoding of the road traffic data, transmission data bits correspondingto the radiated waves from the road antennas 4 a, 4 b must be insynchronism. Where the phase modulation technique, such as QPSK, is usedfor modulation, any frequency difference between the radiated waves fromthe road antennas will entail abnormal operations of an AFC, resultingin bit errors (see, for example, YOICHI SAITOH, “MODULATION/DEMODULATIONIN DIGITAL RADIO COMMUNICATIONS”, Electronic Information CommunicationAssociation, p.119, 1.10-18). Too great frequency difference disablesthe coherent detection and the encoded signals cannot be decoded at all.

[0126] In the stationary station 1, however, the high-frequency signalsgenerated in the base station 6 are distributed to the road antennas 4a, 4 b via the optical fibers 5 a, 5 b and therefore, no differenceoccurs between the radiated waves from the antennas 4 a, 4 b. Byeliminating delay difference between the optical fibers 5 a, 5 b, thetransmission bits may readily be synchronized.

[0127] The characteristic of producing no frequency difference iseffectively exhibited by applying OFDM (Orthogonal Frequency DivisionMultiplex), as modulation technique, to the modulation unit 22. OFDM isa modulation system wherein data is divided and multiplexed usingmultiple carriers orthogonal to each other. OFDM arranges carrierfrequencies at such narrow intervals that any frequency shift will causeinter-carrier interference, leading to serious deterioration ofcommunication quality. Such a drawback may be eliminated by the opticalfiber radio signal transmission system employing the optical fibercoupler 59 for signal distribution because in principle, the carriersradiated from the antennas 4 a, 4 b have the same frequency. Thus, thepresent roadway communication system is provided with the greatestpossible merit of OFDM which is less susceptible to multipathinterference.

[0128]FIG. 5 is a block diagram illustrating an electrical configurationof the vehicle mounted device 3. The vehicle communication unit 11includes a transmission unit 41 for supplying the vehicle data to thestationary station 1. The transmission unit 41 includes a modulationunit 42 which modulates a modulation-carrier based on the vehicle datafor generating a transmission signal. A suitable modulation technique isexemplified by QPSK and the like.

[0129] The transmission signal is supplied to a mixer 43 where it iscombined with a frequency-modulation carrier outputted from a localoscillator 44, thus converted to a radio transmission signal. Thetransmission signal is amplified by a high-frequency amplifier 45 andthen supplied to the vehicle antennas 12 a, 12 b to be radiatedtherefrom as electromagnetic waves.

[0130] The vehicle communication unit 11 further includes the receptionunit 46 for obtaining the road traffic data from the road antennas 4 a,4 b. When the vehicle antennas 12 a, 12 b receive the radiations fromthe road antennas 4 a, 4 b, reception signals corresponding to thereceived waves are applied to the vehicle communication unit 11. Thereception signals are then amplified in high-frequency amplifiers 47 a,47 b so as to be supplied to a switch unit 48 such as comprised of asemiconductor switch. The amplified signals are also applied to a levelcomparator 49 which, in turn, compares the reception levels of thereception signals thereby to determine which of the signals has thehigher reception level. Subsequently, the comparator provides control ofthe switch unit 48 for passage of the reception signal of the highestreception level.

[0131] In the block diagram of FIG. 5, the two signals are switched bythe switch unit 48. However, an alternative configuration is possiblewherein the two signals are weighted with a predetermined weightingfactor and then combined. In this case, “the predetermined weightingfactor” is determined based on the reception levels of the receptionsignals compared by the level comparator 48.

[0132] In the configuration of the block diagram of FIG. 5, the switchunit 48 switch the high-frequency signals amplified by thehigh-frequency amplifiers 47 a, 47 b. However, an alternativeconfiguration may be made such that data detected by a detector 52 areswitched or combined. Otherwise, data decoded by a decoder 53 may beswitched or combined.

[0133] Since the reception unit 46 of the vehicle communication unit 11is adapted to select reception signal based on a so-called antennapattern diversity system, the vehicle data are accurately restored inthe subsequent step.

[0134] The reception signals through the switch unit 48 are applied to amixer 50 where they are combined with a frequency-conversion carrieroutputted from a local oscillator for frequency conversion.Subsequently, the resultant signals are supplied to the detector 52 forcoherent detection using a demodulation carrier. Then, the signals areapplied to the decoder 53 which converts the reception codes to signalscorresponding to the road traffic data.

[0135] Next, the effect of diversity reception will be described. In thevehicle mounted device 3, a so-called multipath environment isestablished because the electromagnetic waves at the same frequency areincident along forward and backward directions of the vehicle 2. In thecase of the conventional signal reception using a single antenna, thereception signals suffer fading with the sharp fluctuations in amplitudeand phase, as shown in FIG. 6a.

[0136] Depending upon the position of the vehicle 2, there may be adifference between a vehicle-to-antenna 4 a distance and avehicle-to-antenna 4 b distance and hence, which leads to propagationtime delay difference between signals received by the vehicle mounteddevice 3. The propagation time delay difference results in theinter-symbol interference. Consequently, the so-called floor erroroccurs because the bit error rate is not improved despite the highreception level of the received waves. The effect is particularlynoticeable in a case where the base station 6 has such a low modulationrate that the propagation time delay difference becomes significantrelative to one bit time.

[0137] Assume, for example, that the cell E is 100 m in longitudinallength along the road and the road antenna 4 a is 10 m in height, amaximum value of the delay spread between the reception signals from therespective road antennas 4 a, 4 b (standard deviation of time delayweighted with the reception level) is at about 50 nsec. If, in thiscase, the vehicle mounted device 3 uses QPSK coherent detection, as adetection technique, the floor error rate is at about 6×10⁻⁴ with themodulation rate of 1 Mbit/sec (see, for example, SHINSI MASAAKI “RADIOWAVE PROPAGATION IN RADIO COMMUNICATIONS”, Electronic InformationCommunication Association, p.213 Feb. 20, 1994).

[0138] On the other hand, FIG. 6b represents the levels of decomposedwaves received from the respective road antennas 4 a, 4 b. In FIG. 6b,the solid line indicates the level of wave received from the first roadantenna 4 a whereas the two-dot-dash line indicates the level of wavereceived from the second road antenna 4 b. FIG. 6b shows that relativelystable reception signals can be obtained by selecting either one of thewaves that has the maximum reception level.

[0139] Accordingly, the vehicle mounted device 3 of the first embodimentis designed to select either one of the reception signals thatcorresponds to the maximum reception level based on the differentdirectivities of the antennas and to subject the signals thus receivedto the detection and decoding processes. This reduces the inter-waveinterference so that the vehicle mounted device 3 can receive the wavesat levels as shown in FIG. 6c. Thus is obviated the effect of themultipath fading. In the selection of either one of the receptionsignals, if the unselected reception signal has a level somewhat lowerthan that of the selected reception signal, the floor error rate may beimproved. More specifically, if the level of the unselected receptionsignal is about 20 db lower than that of the selected reception signal,the floor error rate is improved to about 1×10⁻⁵ with the delay spreaddecreased to about 1/10.

[0140]FIG. 7 is an explanatory diagram of a reception signal selectionprocedure taken by the vehicle mounted device 3. In FIG. 7, sections A-Band D-F each indicate a wave blocking area produced by a large vehicletraveling in side-by-side relation to the vehicle.

[0141] When the vehicle is traveling in proximity of the first roadantenna 4 a (at position indicated by the solid line in FIG. 1), areception level 71 of the radiation from the first road antenna 4 a ishigher than a reception level 72 of that from the second road antenna 4b. Hence, a reception signal corresponding to the radiation from thefirst road antenna 4 a is selected.

[0142] When, in this state, the vehicle 2 reaches a point A to have theradiation thereto blocked by the large vehicle, the reception level 71drops abruptly. At this time, the vehicle mounted device 3 also receivesthe radiation from the second road antenna 4 b so that the receptionlevel 72 becomes relatively higher than that of the radiation from thefirst road antenna. Thus, the vehicle mounted device 3 selects thereception signal corresponding to the radiation from the second roadantenna 4 b. Subsequently, when the vehicle 2 reaches a point B gettingout of the wave blocking area, the reception level 71 becomes relativelyhigher and therefore, the vehicle mounted device 3 selects the receptionsignal corresponding to the radiation from the first road antenna 4 a.

[0143] After the vehicle 2 passes a midportion C of the cell E (atposition indicated by the two-dot-dash line in FIG. 1), the relationbetween the reception levels 71 and 72 is inverted so that the receptionlevel 72 is relatively the higher. Therefore, the vehicle mounted device3 selects the reception signal corresponding to the radiation from thesecond road antenna 4 b. In the wave blocking section between points Dand F, the reception level 71 is relatively the higher during a periodthat the vehicle 2 travels between the points D and F. Hence, thevehicle mounted device 3 selects the reception signal corresponding tothe radiation from the first road antenna 4 a.

[0144] As mentioned supra, the first embodiment provides a single cell Ewith two propagation paths for the waves radiated from the road antenna4. Therefore, the blocking of the waves is prevented even when thevehicle 2 is traveling near the large vehicle such as a truck. Inaddition, the effect of multipath interference may be obviated becausethe vehicle mounted device 3 selectively processes the reception signalof the maximum reception level. As a result, seamless communicationstake place preferably between the vehicle mounted device 3 and the roadantenna 4.

[0145] According to the forgoing description, the pair of road antennas4 a, 4 b forming a single cell E are disposed at opposite ends of eachcell area with respect to the longitudinal direction of the road definedwith the cell E. However, the locations of the road antennas 4 a, 4 bare not limited to the area ends. As shown in FIG. 8 for instance, theantennas may be located at places rather closer to the midportion of thecell E than at the area ends. In the foregoing description, a singlecell E is formed by a pair of road antennas 4 a, 4 b but may be formedby three or more road antennas. Briefly, the road antennas 4 may bevaried in the location and the number so long as they are capable ofapplying the waves to the vehicle 2 in different incoming directions.

[0146] According to the foregoing description, the road antennas 4 a, 4b employ the optical fiber radio signal transmission system therebydirectly converting the reception signals to the optical signals,dispensing with the frequency conversion. Alternatively, the roadantennas 4 a, 4 b may take a procedure such that the reception signalsare down-converted to intermediate-frequency signals and then convertedto the optical signals to be outputted to the optical fibers 5 a, 5 b.This procedure permits the use of a less costly, commonly used laserdiode as the light source for the optical signal, thus contributing tocost reduction.

[0147] If, in this case, a system is used wherein the base station 6outputs a local oscillation signal to the road antennas 4 a, 4 b (see,for example, Japanese Unexamined Patent Publication No.6-141361 (1994),the reception signals converted in the road antennas 4 a, 4 b maysubstantially matched in frequency.

[0148] In the foregoing description, a plurality of vehicle antennas 12a, 12 b with specific directivities are provided and either of thereception signals received by the vehicle antennas are selected based onthe reception levels of the received waves. However, an alternativeconfiguration is also possible wherein, for example, a single vehicleantenna adapted to switch the directivities thereof is provided and isso controlled as to direct either of the road antennas 4 a, 4 b. Ausable vehicle antenna is exemplified by an adaptive array antenna andthe like.

[0149]FIG. 9 illustrates a configuration wherein the adaptive arrayantenna is applied to the vehicle antenna 12. The reference characters12 a and 12 b represent an element antenna, respectively. (Although twoor more element antennas may actually be installed, the descriptiontakes an example for simplicity wherein two element antennas areprovided.) Reception signals obtained from the element antennas 12 a, 12b are fed back to a phase/amplitude control circuit 154. Thephase/amplitude control circuit 154 uses a known adaptive controlalgorithm to calculate weighted amplitude vector and weighted phasevector for the reception of the strongest beam or the achievement of anoptimum directivity of the vehicle antenna 12 (see, for example, H. Krimand M. Viberg, “Two Decades of Array Signal processing Research” IEEESignal processing Magazine, pp.67-94, July 1996 and Sekiguchi andInagaki, “Pattern Synthesis Theory”, Electrical CommunicationAssociation, vol.48, No.4 (April, 1996). Phase/amplitude signalsdetermined by the phase/amplitude control circuit 154 are supplied tophase shifting devices 152 a, 152 b, and amplifiers 153 a, 153 b wherethey are conditioned to optimum phase and amplitude. The referencecharacter 155 represents a combiner circuit.

Second Embodiment (Road Marker)

[0150]FIG. 10 is a conceptual representation of an arrangement of theroadway communication system according to a second embodiment of theinvention. In FIG. 10, like functional portions are represented by likereference characters, respectively.

[0151] In the first embodiment, the vehicle mounted device 3 obviatesthe effects of fading and the like by selecting the reception signalcorresponding to the maximum reception level. In contrast, the secondembodiment obviates the effects of fading and the like by informing thevehicle mounted device 3 of a position on the road at which the maximumreception level is switched.

[0152] More specifically, road markers 61, 62, such as of magnet,color-coded reflector and light-emitting element, are installed in theroad for indication of the position on the road at which the maximumreception level is switched. Specifically, the road markers 61, 62 aredisposed on the road at longitudinal opposite ends and at a midportionof the cell E. In the road marker 61 disposed at the area end, a code isimplemented in the magnetic field direction or color spectrum forindication of that the radiation from the first road antenna 4 a reachesthe maximum reception level at this point. In the road marker 62disposed substantially centrally of the cell E, a code is provided forindication of that the radiations from the first and second antennas 4a, 4 b become equal in the reception level at this point.

[0153] As shown in FIG. 11, the vehicle mounted device 3 includes amarker detection unit 63, such as comprised of a magnetic sensor,photodetector or the like, for detection of the road markers 61, 62; acode identification unit 64 for identification of a code incorrespondence to the road marker 61, 62 detected by the markerdetection unit 63; and a signal selection unit 65 for selectivelypassing either one of the two reception signals that corresponds to themaximum reception level based on the determination made by the codeidentification unit 64.

[0154] The marker detection unit 63 detects the road marker 61 when thevehicle 2 enters the cell E or leaves the cell E. At this time, the codeidentification unit 64 determines that the radiation from the first roadantenna 4 a presents the maximum reception level. As a result, theswitch unit 48 is controlled by the signal selection unit 65 thereby topass the reception signal from the first road antenna 4 a. Thus, whenthe vehicle 2 is at the position indicated by the solid line in FIG. 10,the reception signal from the first road antenna 4 a is selected andsubjected to the detection and decoding processings.

[0155] When, on the other hand, the vehicle 2 passes the midportion ofthe cell E, the marker detection unit 63 detects the road marker 62 suchthat the code identification unit 64 determines that the radiation fromthe second road antenna 4 b presents the maximum reception level.Accordingly, the signal selection unit 65 provides control of the switchunit 48 for passage of the reception signal from the second road antenna4 b. Thus, when the vehicle 2 is at the position indicated by thetwo-dot-dash line in FIG. 10, the reception signal from the second roadantenna 4 b is selected and subjected to the detection and decodingprocessings.

[0156] According to the second embodiment, the reception signalcorresponding to the maximum reception level can be selected withoutmonitoring the reception level for avoiding the fading effect due to theinterference between the first and second road antennas 4 a, 4 b. Thereception signal can be selected by simple processings.

Third Embodiment (Polarized Wave)

[0157] Next, the description of a third embodiment focuses on differencefrom the first embodiment.

[0158]FIG. 12 is a conceptual representation of an arrangement of theroadway communication system according to the third embodiment.

[0159] The first and second road antennas 4 a, 4 b are adapted toradiate the cell E with the electromagnetic waves in the form of apolarized wave A and a polarized wave B, respectively, the waves beingat the same frequency (6 GHz) and orthogonally polarized with respect toeach other. More specifically, the first road antenna 4 a radiates thepolarized wave A in a direction of the hollow arrow whereas the secondroad antenna 4 b radiates the polarized wave B in a direction of thesolid arrow. Accordingly, the electromagnetic waves of the samefrequency and of different polarization characteristics are incident onany point in the cell E in longitudinally forward and backwarddirections with respect to the road. It is noted that the pair of wavesorthogonally polarized with respect to each other include a pair of aright hand circularly polarized wave and a left hand circularlypolarized wave, and a pair of a horizontally polarized wave and avertically polarized wave, and the like.

[0160]FIG. 13 is a conceptual representation of an arrangement of thevehicle mounted device 3. The vehicle mounted device 3 includes thevehicle communication unit 11 and the vehicle antenna unit 12. Thevehicle antenna unit 12 includes a pair of vehicle antennas 12 a, 12 bmounted on the ceiling of the vehicle 2, for example. The vehicleantennas 12 a, 12 b are juxtaposed along an anteroposterior direction ofthe vehicle 2, having orthogonally polarized characteristics withrespect to each other. More specifically, the vehicle antenna unit 12radiates the polarized wave B forwardly and the polarized wave Brearwardly and receives them. This permits the vehicle antennas 12 a, 12b to receive the electromagnetic waves at high power level from the roadantennas 4 a, 4 b which are directive to the cell. In radiation of wavescontaining the vehicle data, the vehicle antennas 12 a, 12 b providerespective radiations of waves orthogonally polarized with respect toeach other toward the respective road antennas 4 a, 4.

[0161] Although FIG. 13 illustrates that the vehicle antennas 12 a, 12 bhave the directivities in the anteroposterior directions of the vehicle2, antennas without the anteropositerior directivities, such asnon-directional antenna, may be used. Briefly, the vehicle antennas 12a, 12 b may have orthogonally polarized characteristics with respect toeach other because even antennas without the anteroposteriordirectivities are capable of providing the polarization diversity effectof the third embodiment.

[0162]FIG. 14 is a block diagram illustrating an electricalconfiguration of the stationary station 1. The block diagram differsfrom the block diagram of FIG. 3 in that the road antennas 4 a, 4 bradiate and receive the electromagnetic waves in the form of polarizedwave A and polarized wave B, respectively.

[0163] Referring to FIG. 14, the following procedure takes place. Thetransmission unit 21 converts radio transmission output signals tooptical signals. The resultant signals are converted to electricalsignals in optic-electrical converters (O/E) 27 a, 27 b of the roadantennas 4 a, 4 b. Subsequently, the electrical signals are radiatedfrom the road antennas 4 a, 4 b as the polarized wave A and polarizedwave B, respectively.

[0164] When the road antennas 4 a, 4 b receive the radiations from thevehicle antennas 12 a, 12 b, reception signals corresponding to thereceived waves are directly converted to optical signals in theelectro-optical converters (E/O) 29 a, 29 b, respectively. The resultantsignals are outputted to the down-cables 5 a, 5 b, respectively, to besupplied to the reception unit 28 of the base station 6.

[0165] The reception unit 28 includes the two optic-electricalconverters (O/E) 30 a, 30 b where the optical signals are converted backto the original reception signals. The reception signals arerespectively amplified by the high-frequency amplifiers 31 a, 31 bbefore applied to the switch unit 32 such as comprised of asemiconductor switch. The amplified reception signals are also suppliedto the level comparator 33. The level comparator 33 compares thereception levels of the reception signals thereby determining which ofthe signals are at the higher reception level. Then, the comparatorprovides control of the switch unit 32 for passage of the receptionsignal with the maximum reception level.

[0166] In the configuration shown in the block diagram of FIG. 13, thetwo signals are switched by the switch unit 32. However, an alternativeconfiguration is possible wherein the two signals are weighted with apredetermined weighting factor and then combined. In this case, “thepredetermined weighting factor” is determined based on the receptionlevels of the reception signals compared by the level comparator 33.

[0167] In the block diagram of FIG. 14, the switch unit 32 operates toswitch the high-frequency signals amplified by the high-frequencyamplifiers 31 a, 31 b. An alternative configuration is possible whereindata detected by a detector 36 are switched or combined. Anotherconfiguration is also possible wherein data decoded by the decoder 37are switched or combined.

[0168] The base station 6 of the third embodiment is adapted to selectthe reception signal based on the combination of the so-called sitediversity technique and the polarization diversity technique.Accordingly, the vehicle data are accurately restored in the subsequentprocessing.

[0169]FIG. 15 is a block diagram illustrating an electricalconfiguration of the vehicle mounted device 3. The diagram differs fromFIG. 5 in that the radio transmission output signals are amplified bythe high-frequency amplifier 45 and then supplied to the vehicleantennas 12 a, 12 b which, in turn, radiate the signals in the form ofpolarized wave A and polarized wave B, respectively.

[0170] The vehicle communication unit 11 further includes the receptionunit 46 for obtaining the road traffic data from the road antennas 4 a,4 b. When the vehicle antennas 12 a, 12 b receive the radiations fromthe road antennas 4 a, 4 b, the reception signals corresponding to thereceived waves are supplied to the vehicle communication unit 11. Thereception signals are amplified by the high-frequency amplifiers 47 a,47 b and then supplied to the switch unit 48 such as comprised of asemiconductor switch or the like. The reception signals are also appliedto the level comparator 49. The level comparator 49 compares thereception levels of the reception signals to determine which of thesignals has the higher reception level. Subsequently, the comparatorprovides control of the switch unit 48 for passage of the receptionsignal with the maximum reception level.

[0171] In the block diagram of FIG. 15, the two signals are switched bythe switch unit 48. However, an alternative configuration is possiblewherein the two signals are weighted with a predetermined weightingfactor and combined. In this case, “the predetermined weighting factor”is determined based on the reception levels of the reception signalscompared by the level comparator 33.

[0172] In the block diagram of FIG. 15, the switch unit 48 switches thehigh-frequency signals amplified by the high-frequency amplifiers 47 a,47 b. However, an alternative configuration is possible wherein datadetected by the detector 52 are switched or combined. Otherwise, datadecoded by the decoder 53 may be switched or combined.

[0173] The reception signals through the switch unit 48 are applied tothe mixer 50 where they are combined with a frequency-conversion carrieroutputted from a local oscillator 51 for frequency conversion.Subsequently, the resultant signals are supplied to the detector 52 forcoherent detection using a demodulation carrier. Then, the signals areapplied to the decoder 53 where they are converted to reception signalscorresponding to the road traffic data.

[0174] In the vehicle mounted device 3, the so-called multipathenvironment is established because the electromagnetic waves at the samefrequency are received along forward and backward directions of thevehicle 2. However, the effect of fading with the sharp fluctuations inamplitude and phase can be obviated because the incoming waves forwardlyand rearwardly of the vehicle 2 have orthogonal polarization planes withrespect to each other.

[0175] Since the operations described with reference to FIGS. 6 and 7may be directly applied to the selection from the two polarizedreception signals, the description thereof is dispensed with.

[0176] As mentioned supra, the third embodiment provides a single cell Ewith two propagation paths for the waves radiated from the road antenna4 while the vehicle mounted device 3 is adapted to select the polarizedreception signal of the maximum reception level and to process it.Therefore, the blocking of waves and multipath interference areprevented even when the vehicle 2 is traveling near the large vehiclesuch as a truck.

[0177] According to the forgoing description, the pair of road antennas4 a, 4 b forming a single cell E are disposed at longitudinal oppositeends of each cell area with respect to the road defined with the cell E.However, the road antennas 4 a, 4 b may be located at places rathercloser to the central portion of the cell E than at the area ends, asdescribed with reference to FIG. 8.

[0178] In the foregoing description, a single cell E is formed by a pairof road antennas 4 a, 4 b but may be formed by three or more roadantennas. Briefly, the road antennas 4 may be varied in the location andthe number so long as they are capable of applying the polarized wavesto the vehicle 2 in different incoming directions.

[0179] In the foregoing description, a plurality of vehicle antennas 12a, 12 b with specific polarization characteristics are provided andeither of the reception signals received by the vehicle antennas isselected based on the reception levels of the received waves. However,an alternative arrangement is also possible wherein, for example, asingle vehicle antenna adapted to switch the polarizationcharacteristics thereof is provided and is so controlled as to receiveeither of the polarized waves depending upon the reception levels of thewaves. A usable vehicle antenna is exemplified by a polarizationadaptive array antenna and the like.

[0180] As a configuration employing the polarization adaptive arrayantenna, the configuration of FIG. 9 is usable wherein the elementantennas 12 a, 12 b are replaced by differently polarized elementantennas (Although two or more element antennas may actually beinstalled, the description takes an example for simplicity wherein twoelement antennas are provided.) (See, for an exemplary configuration ofthe polarization array antenna, Itoh Yasuhiko & ?? Tasuku, “CURRENTSTATE AND TRENDS OF THIN ANTENNAS” Electronic Information CommunicationAssociation, B Vol.J71-B No.11, pp.1217-1227 (November 1988)).

[0181] Now referring to FIG. 9, assume, for example, that the elementantenna 12 a is a horizontal polarization antenna whereas the elementantenna 12 b is a vertical polarization antenna. The phase/amplitudecontrol circuit 154 provides control based on the conventional adaptivecontrol algorithm such that the vehicle antenna 12 has polarizationcharacteristics of providing polarized waves carrying the most powerfulsignals (generally, elliptically polarized waves).

Fourth Embodiment (Road Marker)

[0182]FIG. 16 is a conceptual representation of an arrangement of theroadway communication system according to a fourth embodiment of theinvention. In FIG. 16, like functional portions to those in FIG. 12 arerepresented by like reference characters, respectively.

[0183] The third embodiment obviates the effects of fading and the likeby taking the steps of comparing the reception levels of the polarizedreception waves and selecting the polarized reception signal of themaximum reception level. In contrast, the fourth embodiment obviates theeffects of fading and the like by informing the vehicle mounted device 3of a position on the road at which the reception signal of the maximumreception level is switched.

[0184] More specifically, the road markers 61, 62, such as of magnet,color-coded reflector and light-emitting element, are installed in theroad for indication of the position on the road at which the maximumreception level is switched.

[0185] As shown in FIG. 17, the vehicle mounted device 3 includes themarker detection unit 63 for detection of the road markers 61, 62; thecode identification unit 64 for identification of a code incorrespondence to the road marker 61, 62 detected by the markerdetection unit 63; and the signal selection unit 65 for providingcontrol of the switch unit 48 such that either one of the two receptionsignals that corresponds to the maximum reception level is passed basedon the determination made by the code identification unit 64.

[0186] The marker detection unit 63 detects the road marker 61 when thevehicle 2 enters or leaves the cell E. At this time, the codeidentification unit 64 determines that the radiation from the first roadantenna 4 a presents the maximum reception level. Accordingly, thesignal selection unit 65 provides control of the switch unit 48 forpassage of the reception signal from the first road antenna 4 a. Thus,the reception signal from the first road antenna 4 a is selected andsubjected to the detection and decoding processings.

[0187] When, on the other hand, the vehicle 2 passes the central portionof the cell E, the marker detection unit 63 detects the road marker 62whereby the code identification unit 64 determines that the radiationfrom the second road antenna 4 b presents the maximum reception level.Accordingly, the switch unit 48 is controlled by the signal selectionunit 65 thereby to pass the reception signal from the second roadantenna 4 b. Thus, the reception signal from the second road antenna 4 bis selected and subjected to the detection and decoding processings.

[0188] According to the fourth embodiment, the reception signalcorresponding to the maximum reception level can be selected withoutmonitoring the reception level for avoiding the fading due to theinterference between the first and second road antennas 4 a, 4 b. Hence,the reception signals can be selected by the simple processings.

Fifth Embodiment (OFDM)

[0189]FIG. 18 is a conceptual representation of a configuration of theroadway communication system according to a fifth embodiment of theinvention. This embodiment pertains a micro-cell roadway communicationsystem employing Orthogonal Frequency Division Multiplex (OFDM)technique as a data modulation technique.

[0190] In the mobile communication system using a single carrier, it isa common practice to provide the transmission unit with an equalizerhaving an inverse characteristics of those of the transmission paththereby obviating the effect of inter-symbol interference caused bymultipath delayed waves.

[0191] However, a car travels through the cell at such high speeds thatthe radio frequency energy field presents too sharp fluctuations perunit time for the equalizer to cope with the calculations. Thus, it isimpossible to transmit signals at less than a given transmission errorrate.

[0192] In addition, a large-scale hardware is required for implementingthe equalizer, which results in great power consumption.

[0193] On this account, Orthogonal Frequency Division Multiplex systemless susceptible to the inter-symbol interference due to multipathdelayed waves is employed as the data modulation system.

[0194] In the stationary station 1, a plurality of cells E arecontinuously defined along the road. Near a boundary of individual cellsE with respect to a longitudinal direction of the road, the first roadantenna 4 a and a second road antenna 4 b are installed, which aredirective toward each cell. The first and second road antennas 41, 4 beach radiate the cell E with electromagnetic waves of the same frequency(e.g., in 6 GHz band). More specifically, the first road antenna 4 aradiates the electromagnetic waves in a direction represented by thehollow arrow whereas the second road antenna 4 b radiates theelectromagnetic waves in a direction represented by the solid arrow.Accordingly, the waves of the same frequency are incident on any pointin the cell E in longitudinally forward and backward directions of theroad. Hence, when passing through the cell E, the vehicle 2 receives theelectromagnetic waves incoming from front and from back.

[0195] The road antennas 4 a, 4 b are connected to the base station 6via the optical fibers 5 a, 5 b, respectively. Each optical fiber 5 a, 5b comprises an up-optical cable and a down-optical cable. The opticalfibers reduce signal attenuation as compared with a coaxial cable or thelike used as the transmission line, thus preventing the degradation ofcommunication quality. As a matter of course, the optical fibers 5 a, 5b may be replaced by the coaxial cables.

[0196] The base station 6 applies OFDM modulated signals to the roadantennas 4 a, 4 b via the optical fibers 5 a, 5 b. Hence, the radiationsfrom the road antennas 4 a, 4 b contain the same road traffic data. Thebase station 6 also obtains the vehicle data from the vehicle mounteddevice 3 via the road antennas 4 a, 4 b and properly processes theobtained data.

[0197] The base station 6 shares with neighboring base stations 6 thesame carrier frequency for OFDM modulated waves. Also, the communicateddata contain the same contents. The transmission of signals of the samecontent over the same carrier frequency channel negates the need forchanging the frequency of the vehicle mounted oscillator at transfer ofthe vehicle to the neighboring cell. Hence, the vehicle mounted devicedoes not require a costly oscillator for fast oscillation frequencypulling or multiple oscillators. This contributes to the reduction ofdevice costs and size.

[0198]FIG. 19 is a conceptual representation of a configuration of thevehicle mounted device 3 which includes the vehicle communication unit11 and the vehicle antenna unit 12. The vehicle communication unit 11radiates electromagnetic waves containing the vehicle data via thevehicle antenna unit 12. The vehicle communication unit 11 also receivesthe waves radiated from the road antennas 4 a, 4 b via the vehicleantenna unit 12, thereby obtaining the road traffic data containedtherein for presenting the obtained road traffic data to a driver, forexample.

[0199]FIG. 20 is a block diagram illustrating an electricalconfiguration of the stationary station 1 which includes thetransmission unit 21 for supplying the road traffic data to the roadantenna 4. The transmission unit 21 employs OFDM modulation techniquewherein data is divided into pieces and multiplexed with multipleorthogonal carriers with respect to each other.

[0200] The transmission unit 12 includes a forward error correctionencoder 130, an interleave circuit 131, a differential encoder 132, aninverse Fourier function transformer 133, and an up-converter 134.

[0201] The forward error correction encoder 130 serves to correct blockcoding errors or convolutional coding errors. The error correctioncircuit is an effective means because field strength variations(standing waves) occur in the road which are responsible for irregularchanges in amplitude and phase of the signals received by the travelingvehicle (fading).

[0202] The interleave circuit 131 performs time-interleave processingand frequency-interleave processing used in Digital Audio Broadcasting.

[0203] The differential encoder 132 performs encoding processingpreparatory to demodulation based on extraction of difference from thepreceding signal. When the transmission path becomes instable, theeffect of the instable transmission path can be canceled throughdifferential extraction.

[0204] The inverse Fourier function transformer 133 implements variousfunctions which include translating serial information to parallelinformation through serial-to-parallel converter; performing inverseFourier transform processing; translating inverse Fourier transformedinformation back to serial information; and time compressing the serialinformation and setting a guard time by placing a rear symbol in aforward position.

[0205] The up-converter 134 up-converts the waves to higher radiofrequencies, similarly to the mixer 23.

[0206] The transmission signals up-converted by the up-converter 134 areconverted to optical signals by the electro-optical converter (E/O) 26.The optical signals are distributed via the optical fiber coupler 59 tobe provided as output to the optical fiber couplers 5 a, 5 b. Theoptical signals are transmitted through the optical fibers 5 a, 5 b soas to be converted to electrical signals by the respectiveoptic-electrical converters (O/E) 27 a, 27 b mounted to the roadantennas 4 a, 4 b. Subsequently, the resultant signals are radiated fromthe road antennas 4 a, 4 b.

[0207]FIG. 21 is a graphical representation of symbol transmission basedon OFDM wherein “f” indicates the frequency axis; “t” the time axis; TSthe valid symbol length; and Δt the guard time. The time compressionratio is expressed as (TS+Δt)/TS.

[0208] The embodiment defines the guard time Δt to be longer than a timedelay over the multipath. Because of the longer guard time, a symboloverlap may be ignored in the demodulation of the received signal withlong propagation time delay.

[0209] The time delay over multipath can be determined by taking actualmeasurement at the cell. Otherwise, the time delay may be empiricallydetermined from the size of the cell. Specifically, the time delay isestimated at about 500 nsec in a 100-m long cell.

[0210] As shown in FIG. 20, the base station 6 includes the receptionunit 28 for obtaining the vehicle data via the road antennas 4 a, 4 b.When the radiation from the vehicle antenna unit 12 is received by theroad antennas 4 a, 4 b, reception signals corresponding to the receivedwaves are directly converted to optical signals by the electro-opticalconverters (E/O) 29 a, 29 b. Subsequently, the resultant optical signalsare outputted to the down-cables 5 a, 5 b to be supplied to thereception unit 28 of the base station 6.

[0211] The reception unit 28 includes two optic-electrical converters(O/E) 30 a, 30 b where the optical signals are converted back to theoriginal reception signals. The reception signals are amplified by thehigh-frequency amplifiers 31 a, 31 b and then supplied to the switchunit 32 such as comprised of a semiconductor switch or the like. Theamplified signals are also applied to the level comparator 33. The levelcomparator 33 compares the reception levels of the reception signals todetermine which of the signals has the higher reception level. Then, thecomparator provides control of the switch unit 32 for passage of thereception signal with the maximum reception level. The reception signalthrough the switch unit 32 is applied to the mixer 34 where it iscombined with a frequency-conversion carrier outputted from the localoscillator 35 for frequency conversion. The resultant signal is appliedto the detector 36 where it is subject to coherent detection using ademodulation carrier. The detected signal is applied to the decoder 37for conversion to the reception signal corresponding to the vehicledata.

[0212] According to the block diagram of FIG. 20, the two signals areswitched by the switch unit 32. However, an alternative configuration ispossible wherein the two signals are weighted using a predeterminedweighting factor and then combined. In this case, “the predeterminedweighting factor” is determined based on the reception levels of thereception signals compared by the level comparator 33.

[0213] According to the block diagram of FIG. 20, the switch unit 32switches the high-frequency signals amplified by the high-frequencyamplifiers 31 a, 31 b. However, an alternative configuration is possiblewherein data detected by the detector 36 are switched or combined.Otherwise, data decoded by the decoder 37 may be switched or combined.

[0214]FIG. 22 is a block diagram illustration a configuration whereinthe switch unit 32 is disposed downstream of the detector for selectionof a reception signal to be passed after coherent detection. Morespecifically, after amplified by the high-frequency amplifiers 31 a, 31b, the reception signals are applied to the mixer 34 a, 34 b forfrequency conversion. Then, the signals are coherently detected by thedetectors 36 a, 36 b before applied to the switch unit 32. On the otherhand, the level comparator 33 receives the reception signals amplifiedby the high-frequency amplifiers 31 a, 31 b and controls the switch unit32 in a manner that the either of the reception signals that has themaximum reception level is allowed to pass the switch unit 32.

[0215] The procedure wherein the detection is followed by the selectionof the reception signal is effective to obviate noise introduction intothe reception signals, thus preventing the deterioration ofcommunication quality.

[0216] The foregoing configuration employs the so-called optical fiberradio signal transmission system as a transmission system for outputtingthe optical signals to the optical fibers 5 a, 5 b.

[0217] This negates the need for mounting the transmission/receptionunit to the respective road antennas 4 a, 4, permitting thetransmission/reception unit for the road antennas to take form as oneset mounted to the base station 6. Thus, the road antennas 4 a, 4 b maybe constructed simple. On the other hand, the base station 6 may processthe reception signals fed from the road antennas 4 a, 4 b as they are athigh frequencies. Hence, the level comparator 33 may readily compare thehigh-frequency reception levels of the reception signals.

[0218] The transmission unit 21 employs the OFDM modulation technique.In the OFDM system, carrier frequencies are arranged at narrowintervals. Therefore, any frequency shift will entail the inter-carrierinterference and serious deterioration of the communication qualityresults. This drawback may be eliminated by employing the optical fiberradio signal transmission system where the signals are distributed viathe optical fiber coupler 59. The reason is because the carriersradiated from the road antennas 4 a, 4 b, in principle, have the samefrequency. Thus, the inventive roadway communication system takes fulladvantage of the merit of OFDM technique resisting multipahinterference.

[0219]FIG. 23 is a block diagram illustrating a configuration of thereception unit for receiving the radiations from the road antennas 4 a,4 b via the vehicle antenna 12.

[0220] The reception unit includes a down-converter 140, a Fourierfunction transformer 141, a differential decoder 142, a deinterleavecircuit 143, and a forward error correction decoder 146.

[0221] The Fourier function transformer 141 operates inversely of theinverse Fourier function transformer 133 on the send side. The circuitgenerates a decoded signal by subjecting the received waves to Fouriertransform based on a window length of the effective symbol period TS.

[0222] The differential decoder 142 and the deinterleave circuit 143operate inversely of the differential encoder 132 and the interleavecircuit 131, respectively.

[0223] The forward error correction decoder 146 operates inversely ofthe forward error correction encoder 130.

[0224] As mentioned supra, the fifth embodiment provides a single cell Ewith two propagation paths for the waves radiated from the road antenna4. Therefore, the wave blocking is prevented even when the vehicle 2 istraveling near the large vehicle such as a truck. In addition, theeffect of multipath interference may be obviated by virtue of OFDMtechnique which provides the guard time to avoid the inter-symbolinterference. As a result, the seamless communications take place in apreferable manner between the vehicle mounted device 3 and the roadantenna 4.

[0225] According to the foregoing description, the pair of road antennas4 a, 4 b forming a single cell E are disposed at longitudinal oppositeends of each cell area with respect to road defined with the cell E.However, the locations of the road antennas 4 a, 4 b are not limited tothe area ends. As shown in FIG. 24 for instance, the antennas may belocated at places rather closer to the midportion of the cell E than atthe area ends. In the foregoing description, a single cell E is formedby a pair of road antennas 4 a, 4 b but may be formed by three or moreroad antennas. Briefly, the road antennas 4 may be varied in thelocation and the number so long as they are capable of applying thewaves to the vehicle 2 in different incoming directions.

[0226] According to the foregoing description, the road antennas 4 a, 4b employs the optical fiber radio signal transmission system anddirectly converts the reception signals to the optical signals,dispensing with the frequency conversion. Alternatively, the roadantennas 4 a, 4 b may take procedure such that the reception signals aredown-converted to intermediate-frequency signals and then converted tothe optical signals to be outputted to the optical fibers 5 a, 5 b. Thisprocedure permits the use of a less costly, commonly used laser diode asthe light source for the optical signal, thus contributing to costreduction.

[0227] If, in this case, the system is used wherein the base station 6outputs a local oscillation signal to the road antennas 4 a, 4 b (see,for example, Japanese Unexamined Patent Publication No.6-141361 (1994)),the reception signals converted in the road antennas 4 a, 4 b maysubstantially be matched in frequency.

Sixth Embodiment (Sub-area)

[0228]FIG. 25 is a conceptual representation of an arrangement of theroadway communication system according to a sixth embodiment of theinvention. In the sixth embodiment, like functional portions to those inFIG. 1 are represented by like reference characters, respectively.

[0229] Although the first to fifth embodiments use a pair of roadantennas 4 a, 4 b to define a single cell E for prevention of the waveblocking, the sixth embodiment is designed to obviate the wave blockingby installing a plurality of road antennas 71 in a cell E radiated withthe waves of the same frequency. That is, the cell E is divided intoplural sub-areas Es.

[0230] More specifically, one base station 6 is connected with four roadantennas 71 a, 71 b, 71 c, 71 d via optical fibers 72 a, 72 b, 72 c, 72d (hereinafter, collectively referred to as “optical fiber 72”). Theroad antennas 71 a-71 d each radiate the sub-areas ES with waves at thesame frequency modulated by using the road traffic data of the samecontent.

[0231] In a case, for example, where the vehicle is at positiondownstream of a midportion of the sub-area while a truck running in thenext lane is at position diagonally rearward of the vehicle, the wavesmay be blocked by the truck. However, when the vehicle in thispositional relation with the truck enters the next sub-area Es, thewaves are incident in a different direction or the vehicle receivesincoming waves from front. Therefore, the vehicle mounted device 3 isallowed to receive the radiation to the next sub-area Es despite thetruck running diagonally rearward of the vehicle. That is, the vehiclemounted device 3 is allowed to receive any of the radiations from theroad antennas 71 during the passage through the four sub-areas Es.

[0232] When the vehicle is crossing a boundary between the sub-areas Es,the waves of the same frequency are incident upon the vehicle from frontand back. However, no particular trouble occurs because the vehiclemounted device takes advantage of the diversity reception, selectivelyprocessing the reception signal of the maximum reception level. Besides,the effect of multipath interference can be obviated by using the OFDMmodulation technique which prevents the inter-symbol interference byproviding the guard time.

[0233]FIG. 26 is a block diagram illustrating an electricalconfiguration of the base station 6. The base station 6 includes atransmission unit 81 having a modulation unit 82 in which a transmissionsignal containing the road traffic data is generated. The transmissionsignal is applied to a transmission IF unit 83 and then to anelectro-optical converter (E/O) 84 where it is combined with acarrier-generation reference signal for conversion to an optical signal.The resultant optical signal is distributed by the optical fiber coupler59 to be outputted to the up-cables 72 a-72 d.

[0234] A reception unit 85 of the base station 6 includeselectro-optical converters (E/O) 86 a, 86 b, 86 d, 86 d for convertingoptical signals to reception signals, the optical signals being sentfrom the road antennas 71 a-71 d via the optical fibers 72 a-72 d,respectively. The reception signals are subjected to processings such asamplification in respective reception IF units 87 a, 87 b, 87 c, 87 d.Subsequently, the reception signals are applied to respective detectors88 a, 88 b, 88 c, 88 d for demodulation. Then, the resultant receptionsignals are supplied to a switch unit 89. The signals are also suppliedto a level comparator 90 which compares the reception levels of thereception signals to determine which of the signals has the highestreception level. Then, the comparator provides control of the switchunit 89 for passage of a reception signal of the maximum receptionlevel. The reception signal through the switch unit 89 is decoded by adecoder 91. Thus is generated a decoded signal corresponding to thevehicle data.

[0235] In the configuration shown in the block diagram of FIG. 26, theplural signals are switched by the switch unit 89. However, the signalsmay be weighted with a predetermined weighting factor and then combinedtogether. In this case, “the predetermined weighting factor” isdetermined based on the reception levels of the reception signalscompared by the level comparator 90 .

[0236] In the block diagram 26, the signals detected by the detectors 88a-88 d are switched by the switch unit 89. An alternative configurationis possible wherein IF signals provided by the electro-opticalconverters (E/O) 86 a-86 d are switched or combined together. Otherwise,decoded data from the decoder 91 may be switched or combined together.

[0237]FIG. 27 is a block diagram illustrating an electricalconfiguration of the road antenna 71, to which an antenna communicationassembly 100 is mounted. The antenna communication assembly 100 includesa transmission unit 101 having an optic-electrical converter (O/E) 102which converts an optical signal to a transmission signal and acarrier-generation reference signal, the optical signal sent from thebase station 6 via the optical fiber 72. An output from theoptic-electrical converter (O/E) 102 is sent to a transmission IF unit102 via an unillustrated bandpass filter, so that the transmission IFunit 103 receives the transmission signal alone. After amplified by thetransmission IF unit 103, the transmission signal is supplied to atransmission mixer 104. The output from the optic-electrical converter(O/E) 102 is also supplied to a local oscillator 105 via anunillustrated lowpass filter, so that the local oscillator 105 receivesthe carrier-generation reference signal alone. The local oscillator 105outputs a carrier based on the carrier signal thereby to supply thecarrier to the transmission mixer 104. The transmission mixer 104, inturn, combines the transmission signal and the carrier together togenerate a radio transmission signal. The radio transmission signal issupplied to a high-frequency amplifier 106 where it is subject tofrequency up-amplification before supplied to the road antenna 71 via acirculator 107. Thus, the resultant transmission waves are radiated fromthe road antenna 71.

[0238] The antenna communication assembly 100 also includes a receptionunit 108. The reception unit 108 includes a high-frequency amplifier 109for up-amplifying a reception signal which is received by the roadantenna 71 and sent via the circulator 107. The amplified signal issupplied to a reception mixer 110 where it is combined with the carrierfrom the local oscillator 105 for amplification. Subsequently, theresultant signal is applied to a reception IF unit 111 for frequencyconversion. Then, the signal is converted to an optical signal by anelectro-optical converter (E/O) 112 before outputted to the opticalfiber 72.

[0239] According to the sixth embodiment, the vehicle mounted device 3is allowed to receive the radiation in any of the sub-areas Es becausethe electromagnetic waves of the same frequency, which are OFDMmodulated with the road traffic data of the same content, are dividedfor radiation to the plural sub-areas Es. This ensures the seamlesscommunications between the vehicle mounted device 3 and the road antenna71 in a single cell E.

[0240] Because of a relatively small size of the sub-area Es, the roadantenna 71 requires a small transmission power. This reduces cost forthe road antenna 71.

What is claimed is:
 1. A roadway communication system comprising aplurality of road transmission antennas, and a vehicle mounted devicereceiving electromagnetic waves radiated from the road transmissionantennas, wherein the road transmission antennas are disposed atdifferent places along a road and each radiate the same cell with thewaves carried at the same frequency and containing the same content, andwherein the vehicle mounted device comprises vehicle reception antennashaving different directivities for receiving the waves radiated from theroad transmission antennas, and diversity reception means for performingdiversity reception using these vehicle reception antennas.
 2. Theroadway communication system of claim 1, wherein the vehicle mounteddevice further comprises reception-level detection means for detecting areception level of each directive wave received by the vehicle receptionantenna, and wherein the diversity reception means performs thediversity reception based on the reception level detected by thereception-level detection means.
 3. The roadway communication system ofclaim 1, wherein the diversity reception means performs either of thefollowing operations for the diversity reception: (a) an operation ofswitching or combining the signals which were received by the vehiclereception antennas and are to be decoded; and (b) an operation ofswitching or combining the codes which were received by the vehiclereception antennas and then decoded.
 4. The roadway communication systemof claim 1, wherein the vehicle reception antennas are an array antenna,whereas the vehicle mounted device further comprises reception-signaldetection means for detecting a reception level or phase of the wavereceived by each of the vehicle reception antennas, and wherein thediversity reception means performs the diversity reception usinginformation on the reception level or phase detected by thereception-signal detection means.
 5. The roadway communication system ofclaim 1, further comprising a signal transmission unit for transmittingsignals modulated with data of the same content to the road transmissionantennas via a plurality of transmission lines, wherein an optical fiberradio signal transmission system is used as a transmission system foroutputting the signals to the transmission lines.
 6. The roadwaycommunication system of claim 1 or 5, wherein orthogonal FrequencyDivision Multiplex (OFDM) modulation technique in which a guard time isprovided at each symbol is used as a data modulation technique.
 7. Aroadway communication system comprising a vehicle mounted device, and aplurality of road reception antennas for receiving electromagnetic wavesradiated from the vehicle mounted device, wherein the vehicle mounteddevice comprises vehicle transmission antennas for multiple directionradiation of electromagnetic waves modulated with vehicle data, andwherein the plural road reception antennas are disposed at differentplaces along a road as providing directivity to the same cell, andinclude diversity reception means for performing diversity receptionbased on the signals received by the road reception antennas.
 8. Theroadway communication system of claim 7, further comprisingreception-level detection means for detecting reception levels of theplural road reception antennas, wherein the diversity reception meansperforms the diversity reception based on the reception level detectedby the reception-level detection means.
 9. The roadway communicationsystem of claim 7, wherein the diversity reception means performs eitherof the following operations for the diversity reception: (a) anoperation of switching or combining the signals received by the roadreception antennas; and (b) an operation of switching or combining thecodes which were received by the road reception antennas and thendecoded.
 10. The roadway communication system of claim 7, furthercomprising a signal reception unit for receiving, via transmissionlines, the signals received by the road reception antennas, wherein anoptical fiber radio signal transmission system is used as a transmissionsystem for transmitting the signals through the transmission lines. 11.The roadway communication system of claim 7, wherein the vehicle mounteddevice uses Orthogonal Frequency Division Multiplex (OFDM) modulationtechnique, as a data modulation technique, in which a guard time isprovided at each symbol.
 12. A roadway communication system comprising aplurality of road transmission antennas and a vehicle mounted devicereceiving electromagnetic waves radiated from the road transmissionantennas, wherein the road transmission antennas are disposed atdifferent places along a road and each radiate the same cell with thewaves carried at the same frequency and containing the same content,wherein a position marker is disposed at or near the road for informinga position on the road at which reception levels of the waves radiatedfrom the plural road transmission antennas are switched, and wherein thevehicle mounted device comprises vehicle reception antennas havingdifferent directivities for receiving the waves radiated from the pluralroad transmission antennas, marker detection means for detecting anarrival of the vehicle at the position marker, and reception meansperforming any one of the following operations a-c in response to themarker detection means detecting the arrival of the vehicle at theposition marker: (a) an operation of switching the directivities of thevehicle reception antennas using phase control, (b) an operation ofswitching or combining the signals received by the vehicle receptionantennas, and (c) an operation of switching or combining the codes whichwere received by the vehicle reception antennas and then decoded.
 13. Aroadway communication system comprising a plurality of road transmissionantennas, and a vehicle mounted device receiving electromagnetic wavesradiated from the road transmission antennas, wherein the roadtransmission antennas each have a specific polarization characteristicand radiate the same cell with the waves carried at the same frequencyand containing the same content, and wherein the vehicle mounted devicecomprises a plurality of vehicle reception antennas having differentpolarization for receiving the waves radiated from the road transmissionantennas, and diversity reception means performing diversity receptionusing the vehicle reception antennas.
 14. The roadway communicationsystem of claim 13, wherein the vehicle mounted device further comprisesreception-level detection means for detecting reception levels of thewaves received by the vehicle reception antennas on apolarization-characteristic basis, and wherein the diversity receptionmeans performs the diversity reception based on the reception leveldetected by the reception-level detection means.
 15. The roadwaycommunication system of claim 13, wherein the diversity reception meansperforms either of the following operations for the diversity reception:(a) an operation of switching or combining the signals which werereceived by the vehicle reception antennas and are to be decoded; and(b) an operation of switching or combining the codes which were receivedby the vehicle reception antennas and then decoded.
 16. The roadwaycommunication system of claim 13, wherein the vehicle reception antennasare a polarization array antenna whereas the vehicle mounted devicefurther comprises reception-signal detection means for detecting areception level or phase of the wave received by each of the vehiclereception antennas, and wherein the diversity reception means performsthe diversity reception using information on the reception level orphase detected by the reception-signal detection means.
 17. The roadwaycommunication system of claim 13, further comprising a signaltransmission unit for transmitting signals modulated with data of thesame content to the road transmission antennas via a plurality oftransmission lines, wherein an optical fiber radio signal transmissionsystem is used as a transmission system for outputting the signals tothe plural transmission lines.
 18. The roadway communication system ofclaim 13 or 17, wherein orthogonal Frequency Division Multiplex (OFDM)technique in which a guard time is provided at each symbol is used as adata modulation technique.
 19. A roadway communication system comprisinga vehicle mounted device, and a plurality of road reception antennas forreceiving electromagnetic waves radiated from the vehicle mounteddevice, wherein the vehicle mounted device comprises vehicletransmission antennas with different polarization characteristics forradiating electromagnetic waves modulated with vehicle data, wherein theplural road reception antennas each have a specific polarizationcharacteristic and are disposed to provide directivity to the same cell,wherein the road reception antennas each comprise diversity receptionmeans for performing diversity reception based on the signals receivedby the road reception antennas.
 20. The roadway communication system ofclaim 19, further comprising reception-level detection means fordetecting reception levels of the plural road reception antennas on apolarization-characteristic basis, and wherein the diversity receptionmeans performs the diversity reception based on the reception leveldetected by the reception-level detection means.
 21. The roadwaycommunication system of claim 19, wherein the diversity reception meansperforms either of the following operations for diversity reception: (a)an operation of switching or combining the signals received by the roadreception antennas; and (b) an operation of switching or combining thecodes which were received by the road reception antennas and thendecoded.
 22. The roadway communication system of claim 19, furthercomprising a signal reception unit for receiving, via transmissionlines, the signals received by the road reception antennas, and whereinan optical fiber radio signal transmission system is used as atransmission system for outputting the signals to the transmissionlines.
 23. The roadway communication system of claim 19, wherein thevehicle mounted device uses orthogonal Frequency Division Multiplex(OFDM) modulation technique, as a data modulation technique, in which aguard time is provided at each symbol.
 24. A roadway communicationsystem comprising a plurality of road transmission antennas, and avehicle mounted device receiving electromagnetic waves radiated from theroad transmission antennas, wherein the road transmission antennas aredisposed at different places along a road, each antenna having aspecific polarization characteristic and radiating the same cell withthe waves carried at the same frequency and containing the same content,wherein a position marker is disposed at or near the road for informinga position on the road at which reception levels of the waves radiatedfrom the road transmission antennas are switched, and wherein thevehicle mounted device comprises vehicle reception antennas havingdifferent polarization characteristics for receiving the waves radiatedfrom the road transmission antennas, marker detection means fordetecting an arrival of the vehicle at the position marker, andreception means for performing any one of the following operations a-cin response to the marker detection means detecting the arrival of thevehicle at the position marker: (a) an operation of switching thepolarization characteristics of the vehicle reception antennas usingphase control; (b) an operation of switching or combining the signalsreceived by the vehicle reception antennas; and (c) an operation ofswitching or combining the codes which were received by the vehiclereception antennas and then decoded.
 25. A roadway communication systemcomprising a plurality of road transmission antennas, and a vehiclemounted device receiving electromagnetic waves radiated from the roadtransmission antennas, wherein the road transmission antennas aredisposed at different places along a road and each radiate the same cellwith OFDM (Orthogonal Frequency Division Multiplex)-modulated wavecontaining the same content, wherein the vehicle mounted devicecomprises a vehicle reception antenna for receiving the waves radiatedfrom the road transmission antennas, and reception means fordemodulating the waves received by the vehicle reception antenna. 26.The roadway communication system of claim 25, further comprising asignal transmission unit for transmitting signals modulated with data ofthe same content through a plurality of transmission lines to the roadtransmission antennas, wherein an optical fiber radio signaltransmission system is used as a transmission system for outputting thesignals to the transmission lines.
 27. The roadway communication systemof claim 25 or 26, wherein Orthogonal Frequency Division Multiplex(OFDM) modulation technique in which a guard time is provided at eachsymbol is used as a data modulation technique.
 28. A roadwaycommunication system comprising a vehicle mounted device, and aplurality of road reception antennas receiving electromagnetic wavesradiated from the vehicle mounted device, wherein the vehicle mounteddevice comprises a vehicle transmission antenna for radiating waveswhich are OFDM (Orthogonal Frequency Division Multiplex)-modulated withvehicle data, and wherein the plural road reception antennas aredisposed at different places along a road as providing directivity tothe same cell and each include road reception means for performingdemodulation using a signal received by the road reception antenna. 29.The roadway communication system of claim 28, wherein the road receptionantenna uses an optical fiber radio signal transmission system foroutputting the received signal to a transmission line to the roadreception means.
 30. The roadway communication system of claim 28,wherein the vehicle mounted device uses OFDM modulation technique, as adata modulation technique, in which a guard time is provided at eachsymbol.
 31. The roadway communication system of any one of claims 1 to30, wherein the plural road transmission antennas each define anindividual one of plural sub-areas which are constituting a single cell.32. The roadway communication system of any one of claims 1 to 31,wherein communications are carried out over a plurality of continuouscells, using signals at the same frequency and of the same content. 33.The roadway communication system of any one of claims 1 to 32, whereinthe plural road transmission/reception antennas are disposed near a cellboundary with respect to a longitudinal direction of the road.